Fuses are routinely used in the design of monolithic integrated circuits (IC), and in particular in memory devices as elements for altering the configuration of the circuitry contained therein. As such, memories are commonly built with programmed capabilities wherein fuses are selectively "blown" by, e.g., a laser beam.
It is also well known that random access memories (RAM) are designed with redundancies which include spare columns or rows or even fully functional arrays, wherein when any of these spare elements fails, the defective row, column, and the like are replaced by a corresponding element. Disabling and enabling spare elements is accomplished by fuses which are strategically placed throughout the IC and which are blown (i.e., melted away) when required, preferably, by the laser beam.
Additionally, the technique of laser fuse deleting (trimming) has been widely used both in memory and logic IC fabrication industry, as an effective way to improve functional yields and to reduce development cycle time. Yet, fuse blow yield and fuse reliability have been problematic in most conventional fuse designs.
Fuse elements are routinely built of aluminum, copper, polysilicon, silicide or any high conductive metal or alloy. Generally, two ends of a conductive line are joined by a neck, of a width considerably smaller than that of the conductive lines, so as to require less energy to zap the connection. Practitioners of the art will fully appreciate that a plurality of such fuses may be placed side by side. As such, by requiring less energy to zap one of the fuses, less likelyhood exists that damage may be caused on an adjoinig fuse or a circuit element placed nearby.
Various configurations of such fuse elements have been described. In U.S. Pat. No. 4,682,204 to Shiozaki et al., a fuse element prepared from, for instance, polycrystalline silicon, is deposited on an insulated substrate. The connecting portions are integrally formed at both ends of the fuse link. Each of the connecting portions includes a plurality of stepped sections contacting a corresponding stepped section formed on the insulated substrate. In this manner, by increasing the heat capacity of the connecting portions, the fuse link requires less area, thereby decreasing the overall surface required by the fuse structure.
In another embodiment, described in U.S. Pat. No. 4,198,744, to Nicholay, a suspended fuse element including a first metallic layer on an insulation layer deposited on a substrate, is followed by the formation of a fusible material, wherein the material is selectively removed to define the fuse element having a neck portion. This process has the distinct advantage of lowering the thermal conductivity of the fuse link, thereby lessening the amount of energy required to blow it.
In yet another embodiment described in U.S. Pat. No. 5,321,300 to Usuda et al., a laser-broken fuse is formed on an interlevel insulating film. A polisilicon-made heat member is provided in the interlevel film underneath the fuse link. The heat member is placed on an insulated field. This member generates heat by absorbing energy from the laser beam and thermally explodes, breaking the fuse link.
In addition to optimizing the dimensions of the fuse link to lower the energy required to program a fuse, it has been found advantageous to place a shield-plate under the fuse structure to avert damaging adjoining areas of an IC.
Such an arrangement is described, e.g., in U.S. Pat. No. 5,279,984, to Konoshita, et al., wherein at least a portion of the shield plate is placed directly below the fuse element. Appropriate care needs to be taken to ensure that field shield plate is not short-circuited even when a laser beam is irradiated with a deviation for severing the fuse link. The shield-plate described by Konoshita is preferably made of polysilicon or of an equivalent material, which in its nature, absorbs much of the energy provided by the laser. Practitioners of the art will fully appreciate that in addition to the shielding properties obtained by placing a shield underneath the fuse, it would be far more advantageous to reduce the amount of laser energy required to blow the fuse to minimize the damage which potentially could be caused to other elements within the IC.
Referring to FIG. 1, the top view of a conventional fuse structure is shown with two ends of a conductive line 10, respectively attached to a fuse link 20. Also shown is the optimal position of laser spot 15. The energy transmitted by the laser is lost in the area enclosed by the spot, and only a fraction of the total energy is absorbed by the fuse link 20. As a result, the amount of energy to zap the fuse is considerably higher than that required if most of the energy could somehow be channeled to the fuse link.
FIG. 1 also illustrates a conventional fuse design interacting with a single pulse laser. Normally, a straight fuse link 20 is lined-up in a fuse bank. In special situations, a dummy fuse 18 is inserted between two adjoining fuses to achieve better planarization. The dummy fuse 18 is typically placed in the center of the open spaces between two active fuses. The dummy fuse is nothing more than a section of interconnection within the fuse bank, but electrically segregated from other fuses in the bank. Regardless as to whether the fuse is active or a dummy, conventional fuse designs do not usually achieve their intended purpose, i.e., of achieving optimal fuse blow efficiency. The laser energy distribution can be described by either a Gaussian or a rectangular spatial distribution. Indeed, a conventional fuse typically only occupies one-third or less of the area covered by the laser. Even with a highly coherent laser beam, two thirds or more of the laser energy pulse is still required to couple sufficient energy into the active fuse in order to blow it. More specifically, near 70% of the laser energy is transmitted or reflected in the area not covered by the fuse link 20. Even in the 30% remaining area that is covered by the fuse link, 20-60% of the energy is either reflected or transmitted due to multilayer interference effect. Thus, only 12-24% of the total energy is actually used for zapping the fuse. As a result, substantial damage to the substrate can easily occur on areas not covered by the fuses, due to the high rate of absorption and high percentage of laser transmitted, in addition to the higher amount of laser energy that is required to zap the fuse link.
Practitioners of the art will fully appreciate that the width of a fuse cannot be arbitrarily increased. Consistency between the material used and the energy expended to achieve melting of the fuse link must be maintained. Clearly, a link that is too wide may require too high level of energy which may be injurious to the remaining elements forming the IC. 0n the other hand, too little energy may be insufficient to blow the link and explode the protective layer of oxide.